This invention pertains to an ink set for inkjet printing, in particular to an ink set wherein all the inks are mutually interactive so as to reduce bleed of one color into another. The invention also pertains to a method of inkjet printing with this mutually interactive ink set.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on print media, such as paper, to form the desired image. The droplets are ejected from a printhead in response to electrical signals generated by a microprocessor.
Inkjet printers offer low cost, high quality printing and have become a popular alternative to other types of printers. However, inkjet printers are presently unable to match the speed of laser printers, for example, due in large part to the relatively slow dry time of inkjet inks as well as slow-down due to bleed control algorithms in the printer software.
Bleed is a particular problem in inkjet printing because the relatively low viscosity inks used therein tend to spread, and because ink jet printers have the capability of printing three or four primary colors in simultaneous (or near simultaneous) fashion. Bleed is manifested in a mixing or “bleeding” of two printing liquids at their interface, whereby the line of demarcation between the two printing liquids is obscured. Bleeding may cause undesired color formation at the interface and a concurrent loss of resolution, color separation, edge acuity and color purity in the image. The more contrasting the two adjacent liquids are in color (such as black and yellow), the more visible the bleed. Bleed is also particularly noticeable when the mixing of two inks produces a secondary color, such as when blue and yellow mix to produce green.
Various solutions to the bleed problem have been proposed. Some methods involve changing the ink environment to reduce bleed. For instance, heated platens and other heat sources, along with specially formulated paper, have been employed to reduce bleed. However, these add cost, and bleed is preferably controlled without such means.
Another method, and the one most commonly used, involves a delay in applying the second printing liquid until the first printing liquid is completely dry. This method is also disadvantageous, not only because of its inefficiencies, but also because it is not particularly effective. For example, it has been observed that bleed may occur even if the first printing liquid is dry, which is believed to be caused by the colorants of the first printing liquid becoming “redissolved” in the liquid medium of the second printing liquid. Thus, the more soluble the components of the first printing liquid in the liquid medium, the more likely bleed will occur even if the first printing liquid is dry. This method is particularly disadvantageous in ink jet printing applications because it places an arbitrary limitation on the speed of generating multi-colored prints.
Another proposed method for reducing bleed is to apply the two printing liquids at a distance from one another such that no intermingling or mixing of the printing liquids can occur. This method is not an effective solution to the problem, however, and produces images having poor resolution.
Other proposed solutions to the problem of bleed involve changing the ingredients of the ink to include, for example, surfactants or other penetrants. These ingredients can reduce bleed, in certain formulations, but edge acuity and optical density usually suffer.
Some methods are directed at reducing two-way bleed from, for example, black-to-color and especially black-to-yellow. Although these two-way combinations are helpful to a degree, preferably bleed is reduced or eliminated in all combinations of colors in the set.
U.S. Pat. No. 5,700,317, U.S. Pat. No. 6,022,908 and U.S. Pat. No. 6,460,987 all disclose a four-color ink set wherein all inks in the set are designed to interact and thereby reduce bleed. However, these references disclose ink sets wherein one or more of the colorants is a soluble dye (the ink being a solution of the dye), and further disclose no more than the use of four colors. Preferably, for improved light fastness, an interactive ink set can be comprised entirely of pigment colorants. And preferably, for expanded gamut, an interactive ink set can be comprised of more than four colors.
The disclosures of all of the above-identified publications are incorporated by reference herein for all purposes as if fully set forth.
Accordingly, a need still exists for inkjet ink sets that can provide improved print quality (especially, reduced bleed between all colors) and improved edge-acuity while also allowing rapid print speed.